Various resin films such as a polarizer protective film are used for a liquid crystal display device.
In related art, a triacetyl cellulose (TAC) film has been mainly used for the polarizer protective film. However, the TAC film exhibits large moisture permeability and when the TAC film is, in particular, a thin film, it may cause a reduction in the quality of polarizers.
The use of a non-crystalline thermoplastic resin film including a (meth)acrylic resin, a cyclic olefin resin, or a maleimide resin as a main component has been discussed as a new polarizer protective film instead of the TAC film.
For example, an optical film in which resin such as a carbonate resin is added to a (meth)acrylic resin including a copolymer of an alkyl (meth)acrylate monomer and an aromatic vinyl monomer and/or an imide-based monomer is disclosed (e.g., Claim 10 of Patent Literature 1 and Claim 1 of Patent Literature 2). This Patent Literature discloses that the optical film can be preferably used for a polarizer protective film or the like.
The aforementioned non-crystalline thermoplastic resin film tends to exhibit brittleness and reduced mechanical properties when the thickness of the film is reduced. One known process for increasing the toughness of the non-crystalline thermoplastic resin film is a stretching process.
Patent Literature 1 and 2 propose biaxially stretching a thermoplastic resin film at or around a glass transition temperature Tg of the material, more specifically, at a temperature of (Tg−20° C.) to (Tg+30° C.) (paragraph 0046 of Patent Literature 1 and paragraph 0040 of Patent Literature 2).